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United States Patent |
6,153,826
|
Kenny
,   et al.
|
November 28, 2000
|
Optimizing lan cable performance
Abstract
A method of constructing twisted pair cables having an average impedance of
no less than 97.5.OMEGA. and no more than 102.5.OMEGA. is disclosed. The
longest lay length pair is used as a base reference and the construction
of each additional twisted pair is altered to better match the averaged
impedance. Specifically, the insulated conductor thickness T.sub.i of each
twisted pair is adjusted, dependent upon the configuration of the base
pair.
Inventors:
|
Kenny; Robert D. (Canton, MI);
Dickman; Jim (Sidney, NE)
|
Assignee:
|
Prestolite Wire Corporation (Port Huron, MI)
|
Appl. No.:
|
322857 |
Filed:
|
May 28, 1999 |
Current U.S. Class: |
174/27; 174/113R |
Intern'l Class: |
H01B 011/04 |
Field of Search: |
174/113 R,121 A,27,36
|
References Cited
U.S. Patent Documents
5010210 | Apr., 1991 | Sidi et al. | 174/113.
|
5399813 | Mar., 1995 | McNeill et al.
| |
5424491 | Jun., 1995 | Walling et al.
| |
5493071 | Feb., 1996 | Newmoyer | 174/113.
|
5564268 | Oct., 1996 | Thompson.
| |
5576515 | Nov., 1996 | Bleich et al. | 174/113.
|
5597981 | Jan., 1997 | Hinoshita et al.
| |
5734126 | Mar., 1998 | Siekierka et al.
| |
5744757 | Apr., 1998 | Kenny et al.
| |
5770820 | Jun., 1998 | Nelson et al.
| |
5814768 | Sep., 1998 | Wessels et al.
| |
5821466 | Oct., 1998 | Clark et al.
| |
Foreign Patent Documents |
WO 97/39499 | Oct., 1997 | SE.
| |
WO 99/00879 | Jan., 1999 | SE.
| |
Primary Examiner: Kincaid; Kristine
Assistant Examiner: Nguyen; Chau N.
Attorney, Agent or Firm: Rader, Fishman & Grauer PLLC
Claims
What is claimed is:
1. A method of designing a data transmission cable having at least three
twisted pairs, each twisted pair having a unique twist lay length,
comprising:
identifying the unique twist lay length of each twisted pair;
identifying the insulated conductor thickness of the twisted pair having
the longest lay length; and
determining different insulated conductor thicknesses of each remaining
twisted pair solely as a function of the longest lay length to limit
variation of average impedance between the twisted pairs.
2. A method as recited in claim 1, wherein the remaining conductor
thicknesses are determined according to the following relationship:
T.sub.i =XY.sub.i.sup.1/Z,
where
X=insulation thickness of the longest twist lay length pair;
Y.sub.i =the twist ratio of the i.sup.th twist pair;
where 2.ltoreq.Z.ltoreq.10; and where the twist ratio Y.sub.i is found as
follows:
##EQU5##
where L=the twist lay length, measured in inches, of the longest twist lay
length pair; and
L.sub.i =the twist lay length, measured in inches, of the i.sup.th twist
lay length pair.
3. The method of claim 2, wherein Z has a value of between 3 and 5,
inclusive.
4. The method of claim 2, wherein the variation of average impedance
between the pairs is approximately three percent.
5. The method of claim 4, wherein the average impedance is 100.OMEGA. and
the variation of average impedance is .+-.2.5.OMEGA..
6. The method of claim 3, wherein i=4.
7. The method of claim 2, wherein i=4.
8. A data transmission cable, comprising:
at least three twisted pairs, each twisted pair having a unique twist lay
length and a unique insulated conductor thickness, wherein a determination
of said unique insulation conductor thickness for each twisted pair is
predetermined solely as a function of the longest twist lay length to
limit variation of average impedance between said twisted pairs.
9. A data transmission cable as recited in claim 8, wherein said function
obeys the following relationship:
T.sub.i XY.sub.i.sup.1/Z,
where
X=insulation thickness of the longest twist lay length pair;
Y.sub.i =the twist ratio of the i.sup.th pair;
where 2<Z<10; and where the twist ratio Y.sub.i is found as follows:
##EQU6##
where L=the twist lay length, measured in inches, of the longest twist lay
length pair; and
L.sub.i =the twist lay length, measured in inches, of the i.sup.th twist
lay length pair.
10. A data transmission cable as recited in claim 9, wherein said variation
of average impedance is limited to approximately three percent.
11. The method of claim 10, wherein the average impedance is 100.OMEGA. and
the variation of average impedance is .+-.2.5.OMEGA..
Description
FIELD OF THE INVENTION
The present invention relates to a cable made of twisted wire pairs. More
particularly, this invention relates to a twisted pair communications
cable designed for use in high-speed data communications applications.
BACKGROUND OF THE INVENTION
A twisted pair cable includes at least one pair of insulated conductors
twisted about each other to form a two-conductor group. When more than one
twisted pair group is bunched or cabled together, it is referred to as a
multi-pair cable. In certain communications applications using a
multi-pair cable, such as in high speed data transmission, problems are
encountered if the signal transmitted in one twisted pair arrives at its
destination at a different time than the signal transmitted at the same
time by another twisted pair in the cable. In addition, when two or more
wire pairs of different impedance are coupled together to form a
transmission channel, part of any signal transmitted thereby will be
reflected back to the point of attachment. Reflection due to impedance
mismatch between twisted pairs bundled as a multi-pair cable results in
undesired signal loss and unwanted transmission errors, greatly
compromising the speed of data transmission.
To counteract electrical coupling (i.e. "crosstalk") between twisted pairs
of wires bundled as a multi-pair cable, it is known to bundle the twisted
pairs wherein each pair within the multi-pair cable requires a different
distance, called a "twist lay length", to completely rotate about its
central axis. Twist lay length also affects impedance, by affecting both
the capacitance and inductance of the cable. Inductance is proportional to
the distance between paired conductors taken along the lengths of the
conductors, while capacitance in a cable is partially dependent upon the
length of the cable. As may be appreciated, when a cable is constructed
with small twist lay lengths to its twisted pairs, and the twist lay
lengths differ from pair to pair within the multi-pair cable in order to
minimize crosstalk, the changes in twist lay length from pair to pair are
accompanied by large variations in the physical spacing between individual
wires within the pair, thereby affecting inductance. Moreover, if every
pair includes a different twist lay length, then the helical lengths of
each pair of conductors vary widely, thereby affecting capacitance.
Impedance matching within a given multi-pair cable is critical to achieving
high-speed data transmission. However, because the inductance and
capacitance changes from pair to pair within a given multi-pair cable, a
nominal characteristic or "averaged" impedance may be uncontrolled from
pair to pair. In fact, within all cables heretofore known, there is a
tendency for the averaged impedance of at least some pairs within a
multi-pair cable, where the pairs all have small but different twist lay
lengths, to be at or beyond an industry acceptable value.
Currently, the industry accepted value (based upon TIA/EIA 568A-1) for
averaged impedance between twisted pairs is 100 ohms, plus or minus 15%
(100.OMEGA..+-.15.OMEGA.). For example, in a four-pair multi-pair cable,
each of the four pairs must have an average impedance within the
industry-accepted values. Thus, impedance between pairs may vary by up to
30.OMEGA., or by about 27%.
As data transmission speeds have approached the gigabyte per second level,
now achievable due to recent advances in various communications
technologies, the variation between twisted pair averaged impedance within
a multi-pair cable has been found to greatly affect data transmission
performance. Therefore, current industry standards established for lower
data transmission speeds are inadequate. Instead, at these required data
flow levels, actual transmission speed is only achieved when averaged
impedance variation is no less than 97.5.OMEGA. and no greater than
102.5.OMEGA. (100.OMEGA..+-.2.5.OMEGA.).
Thus, numerous attempts have been made within the industry to minimize
differences between twisted pair averaged impedance within a multi-pair
cable, at best by experimentally altering the insulation thickness. In one
attempt, a cable is constructed having multiple twisted pairs divided into
two groups of twisted pairs. The insulation thickness of the two groups is
empirically optimized to a set value within each group of twisted pairs,
and each twisted pair has a different twist lay length. However, even a
minor modification often requires extensive and time-consuming additional
experimentation to find an acceptable cable construction to accommodate
the modification.
In another attempt to minimize averaged impedance, the wires within a
twisted pair are joined along their length, thereby limiting an average
center-to-center distance between wires within a twisted pair along its
length in an attempt to limit inductance effects. Other methods also
attempt to modify a single physical property between the twisted pairs,
including by modifying the chemical composition of the insulating
material, providing special chemical additives to the insulating material,
and by adjusting both insulation thickness and insulation density.
SUMMARY OF THE INVENTION
The present invention is directed to a method of constructing twisted pair
cables having an average impedance of no less than 97.5.OMEGA. and no more
than 102.5.OMEGA. (100.OMEGA..+-.2.5.OMEGA.). In particular, the method of
the present invention focuses on designing and constructing multi-pair
cable from a plurality of twisted pairs wherein each twisted pair has a
different twist lay length.
According to the method of the present invention, the longest lay length
pair is used as the base reference and the construction of each additional
twisted pair is altered to better match the averaged impedance.
Specifically, the insulated conductor thickness T.sub.i of each twisted
pair is determined from the following relationship:
T.sub.i =XY.sub.i.sup.1/Z,
where
X=insulation thickness of the longest twist lay length pair;
Y.sub.i =the twist ratio of the i.sup.th pair; and
where 2.ltoreq.Z.ltoreq.10.
The twist ratio Yi found as follows:
##EQU1##
where L=the twist lay length, measured in inches, of the longest twist lay
length pair; and
Li=the twist lay length, measured in inches, of the i.sup.th twist lay
length pair.
Design and construction of a multi-pair cable according to the present
invention recognizes that average impedance is a very important physical
characteristic of the cable. By maintaining average impedance between
97.5.OMEGA. and 102.5.OMEGA., network throughput is maximized, while data
mismatch problems are significantly reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and inventive aspects of the present invention will become
more apparent upon reading the following detailed description, claims, and
drawings, of which the following is a brief description:
FIG. 1 is a cutaway perspective view of a communications cable.
FIG. 2 is an isolation view of a single twisted pair of wires.
FIG. 3 is an exploded side view of four twisted pairs that comprise a first
embodiment of the invention.
FIGS. 4a-4d show average impedance of the wires of FIG. 3 before
application of the present invention.
FIGS. 5a-5d show average impedance of the wires of FIG. 3 after the
application of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
With reference to FIG. 1, so-called category 5 wiring of the type used for
Local Area Networks (LANs) typically comprises a plurality of twisted
pairs 20 of insulated conductors. In FIG. 1, only two pairs 22, 24 are
shown encased by a jacket 26. Most typically, category 5 wiring consists
of 4 individually twisted pairs, though the wiring may include greater or
fewer pairs as required. For example, wiring is often constructed with 9
or 25 twisted pairs. The twisted pairs may optionally be wrapped in foil
shielding 28, but twisted pair technology is such that most often the
shielding 28 is omitted.
Each twisted pair, shown in FIG. 2, includes a pair of wires 30, 32. Each
wire 30, 32 includes a respective central conductor 34, 36. The central
conductors 34, 36 may be solid metal, a plurality of metal strands, an
appropriate fiberglass conductor, a layered metal, or a combination
thereof. Each central conductor 34, 36 is surrounded by a corresponding
layer 38, 40 of dielectric or insulative material. The diameter D of the
central conductors 34, 36, expressed in AWG size, is typically between
about 18 to about 40 AWG, while the insulation thickness T is typically
expressed in inches (or other suitable units). The insulative or
dielectric material may be any commercially available dielectric material,
such as polyvinyl chloride, polyethylene, polypropolylene or
fluoro-copolymers (like Teflon.RTM.) and polyolefin. The insulation may be
fire resistant as necessary. To reduce electrical coupling or crosstalk
between the wires that comprise a pair, it is known to form each twisted
pair within the cable to have a unique twist lay length LL. Twist lay
length LL is defined as the amount of distance required for the pair of
insulated conductors to completely rotate about a central axis. The
insulation thickness T and the central conductor diameter D combine to
define an insulated conductor thickness T.sub.i. As can be appreciated,
the insulated conductor thickness T.sub.i may be increased or decreased by
changing the value of T, D or both.
The signal attenuation in the insulated conductors is partly dependent upon
the length of the conductors and also upon the distance between them. As a
result, if over a unitary length of cable the twist lay length of one pair
is smaller than for other pairs, then each conductor length in the short
twist lay length pair is longer than in the other pairs. Thus, the short
twist lay length pair tends to attenuate a data transmission signal more
than the other pairs. Moreover, those conductors with the shorter twist
lay length tend to be crushed closer together than other pairs, thereby
bringing the conductors within the pair closer together. In fact, as the
two insulated conductors are twisted together, the insulated conductor
thickness T.sub.I. may be reduced due to the tightness of the twist,
thereby reducing the distance between the central conductors. Undesirably,
reducing the center-to-center distance between the conductors also
increases the attenuation, while at the same time lowering the impedance.
In fact, the impedance decreases rapidly from pair to pair as the twist
lay length becomes shorter.
Thus, the twist lay length LL affects the averaged impedance of each pair
of insulated conductors, and the longer the twist lay length LL, the
higher the impedance.
FIG. 3 shows an example of four twisted pairs 42, 44, 46 and 48 that may
comprise an unshielded twisted pair cable. As discussed above, to decrease
coupling, or crosstalk, between the pairs, each twisted pair is formed
with a different twist lay length. Under ordinary cable construction
methods, the fact that conductor pairs 42, 44, 46 and 48 include different
twist lay lengths means that the averaged impedance between the two
conductors differs. In particular, inductance and capacitance, two factors
that influence average impedance, vary between twisted pairs of different
twist lay lengths. The present invention counteracts the effect of twist
lay length on average impedance, thereby minimizing the average impedance
and significantly improving network throughput.
According to the present invention, the longest lay length pair (reference
42 in FIG. 3) is used as the base reference, and the construction of the
other pairs within a given cable is altered to achieve matched impedances.
For the purposes of illustration only, it will be assumed hereinafter that
a cable having four twisted pairs is to be constructed utilizing the
inventive method. However, it should be understood that the present
inventive method may be applied to cables comprising any number of twisted
pairs to match averaged impedance levels within the cable.
FIGS. 4a-4d show measured averaged impedance of the wires of FIG. 3 before
application of the present invention for purposes of illustrating the
effect of twist lay length on impedance. In FIGS. 4a-4d, impedance (in
.OMEGA.) is plotted as a function of frequency (in MHz) for each of the
pairs shown in FIG. 3, assuming that each pair include 24 AWG conductors
having the twist lay lengths as indicated in column 2 of Table 1. The
measured average impedance values are shown in column 4 of Table 1.
TABLE 1
______________________________________
Average impedance is shown as a function of twist lay length.
Twist Lay
Ref. Length FIG. Average
Number (in.) Number Impedance (.OMEGA.)
______________________________________
42 0.87 3c 104
46 0.74 3d 101
48 0.58 3b 97
44 0.49 3a 96
______________________________________
The cable described in FIGS. 4a-4d and in Table 1 technically meets the
industry-accepted standard set forth in TIA/EIA 568A-1 for averaged
impedance. As noted above, the industry accepted standard requires
averaged impedance within a multi-pair cable to be 100 ohms, plus or minus
15% (100.OMEGA..+-.15.OMEGA.). As shown in FIG. 4 and in Table 1, the
industry standard is relatively easy to meet simply by varying the twist
lay lengths. However, for multi-pair cables including more than four
twisted pairs, it becomes progressively more difficult to match averaged
impedance values for larger numbers of pairs where each pair has a unique
twist lay length.
Moreover, it has been found that the industry accepted standard
(100.OMEGA..+-.15.OMEGA.) is not stringent enough, especially as applied
to extremely high speed data transmission cables (i.e. gigabyte per second
or greater). As applied to gigabyte per second data transmission cables
(and even slower speed transmission cables), small variations between
twisted pair averaged impedance within a multi-pair cable will greatly
affect data transmission performance. The present invention may be used to
optimize transmission levels in all cables, but especially in cables
reaching the gigabyte per second transmission speeds.
It has been found that network performance is optimized when averaged
impedance between pairs in a multi-pair cable is no less than 97.5.OMEGA.
and no greater than 102.5.OMEGA. (100.OMEGA..+-.2.5.OMEGA.). Rather than
empirically determine the physical properties of each twisted pair having
a unique twist lay length, it has been discovered that, by meeting the
following relationships, a multi-pair cable may be constructed including
unique twist lay lengths between each twisted pair having an averaged
impedance of 100.OMEGA..+-.2.5.OMEGA..
Specifically, the insulated conductor thickness T.sub.i of each twisted
pair is found as a function of the insulation thickness of the longest
twist lay length pair in the multi-pair cable as follows:
T.sub.i =XY.sub.i.sup.1/Z, (1)
where
X=insulation thickness of the longest twist lay length pair;
Y.sub.i =the twist ratio of the i.sup.th pair; and
where 2.ltoreq.Z.ltoreq.10.
As noted, the value of Z may be between 2 and 10, inclusive, but most
preferably, Z lies between 3 and 5, inclusive. In addition, the insulated
conductor thickness may be adjusted by increasing the diameter D of the
central conductor, and correspondingly decreasing the insulation thickness
of the longest twist lay length.
The twist ratio Y.sub.i is found as follows:
##EQU2##
where L=the twist lay length, measured in inches, of the longest twist lay
length pair; and
L.sub.i =the twist lay length, measured in inches, of the i.sup.th twist
lay length pair.
EXAMPLE 1
Given the twist lay lengths of the pairs as described above in Table 1, if
the insulated conductor thickness of pair 42 is 0.0065 inches, what
insulated conductor thicknesses for pairs 44, 46 and 48 would optimize
network performance and maintain averaged impedance of
100.OMEGA..+-.2.5.OMEGA.?
Pair 42 has the longest twist lay length, so pair 42 becomes the base
reference. As a first step, twist lay length ratios must be determined
according to Equation 2:
##EQU3##
Applying a midrange Z value of 4 to Equation 1 produces the following:
##EQU4##
FIGS. 5a-5d show measured averaged impedance of the wires constructed
according to Example 1. In FIGS. 5a-5d, impedance (in .OMEGA.) is plotted
as a function of frequency (in MHz) for each of the pairs constructed as
in Example 1. The measured average impedance values are shown in column 4
of Table 2.
TABLE 2
______________________________________
Average impedance of the wires constructed in accordance with
the present invention as calculated in Example 1.
Twist Lay
Ref. Length FIG. Average
Number (in.) Number Impedance (.OMEGA.)
______________________________________
42 0.87 4c 101
46 0.74 4d 100
48 0.58 4b 99
44 0.49 4a 100
______________________________________
As seen in FIGS. 5a-5d, the average impedance over the entire spectrum of
expected frequencies is easily maintained within the target of
100.OMEGA..+-.2.5.OMEGA.. Thus, by applying equations 1 and 2 to shielded
and unshielded cables having any number of twisted pairs, each with a
unique twist lay length, average impedance may be predicted. Design of a
high performance multiple pair cable is therefore as simple as designing a
first twisted pair having a desired impedance, and then applying the
inventive method to as many additional twisted pairs as desired.
Design and construction of a multi-pair cable according to the present
invention recognizes that average impedance is a very important physical
characteristic of the cable. Multi-pair cables constructed according to
the invention maintain the average impedance of the final product to no
less than 97.5.OMEGA. and no more than 102.5.OMEGA.
(100.OMEGA..+-.2.5.OMEGA.). By maintaining average impedance between
97.5.OMEGA. and 102.5.OMEGA., network throughput is maximized, while data
mismatch problems are significantly reduced.
Preferred embodiments of the present invention have been disclosed. A
person of ordinary skill in the art will realize, however, that certain
modifications and alternative forms will come within the teachings of this
invention. Therefore, the following claims should be studied to determine
the true scope and content of the invention.
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